1. Field of the Invention
This invention relates to rotary air filters which can be applied to all types of devices which have to operate in a dusty atmosphere.
For example, such devices may be cooling systems for internal combustion engines or cooling systems for the working fluid of hydraulic equipment. One particularly useful application of the present invention is to the cooling system of the internal combustion engine of a combine harvester because such machines often work in a very dusty atmosphere. Indeed, these machines can only harvest when the crop is ripe and dry which means that during the harvesting operation a considerable amount of dust, chaff and short straw particles are displaced in the vicinity of the machine.
2. Description of the Prior Art
One of the problems with rotary air filters is that either the filter element tends to become blocked relatively quickly when operating in a dusty atmosphere or foreign matter passes through the filter element and tends to block the radiator in the case of a cooling system for an internal combustion engine. In order to meet this problem, rotary air filters have been provided with stationary shields which cover portions of the filter elements as they move relative thereto and thereby interrupt the flow of air through the filter elements, whereby any foreign matter adhering to the elements under suction tends to fall therefrom. An air filter of this type is disclosed in U.S. Pat. No. 3.816.981. More specifically such a filter comprises a perforated rotatable filter element and a fan operable to draw air through the filter element except for that portion covered by the stationary shield at a given instant. A certain proportion of foreign matter drawn against the exterior surface of the filter element is immediately discharged therefrom under centrifugal force which is advantageous. Any foreign matter not removed under centrifugal force will tend to be removed by the action of the stationary shield as already described. Also, because the air filter rotates, there is less chance of foreign matter passing through the perforations in the filter element than would be the case if the filter element were stationary.
Experience with these rotary filters has shown that irrespective of all precautions taken, foreign matter still enters the filter element and in the context of combine harvesters, the most problematical foreign matter is that of barley beards or awns which have an elongated shape. The lengthwise dimensions of the beards or awns exceed the diameter of the perforations in the filter element but the diameters thereof are smaller than the diameter of the perforations. It is assumed that the air turbulences adjacent the outer surface of the filter element are responsible to a great extent for the fact that foreign matter still enters the filter. Experience has also shown that, when the circumferential speed of the rotary filter element exceeds a certain threshold, any foreign matter which enters the element no longer moves further in the direction of the air stream but instead, remains on the inner side of the element, while centrifugal forces obviously have an influence on this phenomenon, an exact and complete explanation cannot be offered.
Furthermore, measurements have indicated that the suction force on the intake side of the fan increases as the distance from the fan decreases, whereby there is a tendency for foreign matter entering the filter element to be drawn towards the fan. If there is any obstruction on the interior surface of the filter element against which foreign matter can build up, the likelihood is that this will result in foreign matter entering the airstream and being taken to the item being cooled, such as a radiator.
The main object of the present invention is to avoid or to attenuate the aforementioned disadvantages of known rotary air filters.